1. Field of the Invention
The invention relates in general to a ballast of a high intensity discharge (HID) lamp driver. More particularly, the invention relates to a high-efficiency electronic ballast with wide input voltage range, high power factor, low current total harmonic distortion (THD) and low current crest factor.
2. Description of Related Art
In the design of the circuit for driving a high intensity discharge (HID) lamp, the HID ballast is an important component of a HID lamp. The performance of the ballast directly affects the starting, the color temperature, the life period and the working stability of the HID lamp. The electronic HID ballast can overcome the disadvantages caused by the iron core HID ballast. Moreover, the ratio of the loading power transmission to the weight of the ballast is high, and the ratio of the power consumption to the controlled power is low. However, the disadvantages of the electronic HID ballast are that the starting of the electronic HID ballast is complex, the power control and the circuit protection are hard to design.
I. Basic Concept and Circuit of a Conventional Ballast of HID Lamp
In general, a conventional basic switching power supply can be classified into three types, one is a flyback and forward converter switching power supply, another is a push-pull switching power supply, and the other is a half bridge/full bridge switching power supply. First, the power loading of the flyback and forward converter switching power supply is small, and the duty cycle is only about 0.4. The use of the flyback and forward converter switching power supply in a HID ballast is described, for example, in the paper of E. Deng and S. Cuk, Single Stage High Power Factor Lamp Ballast, APEC'94, p. 441-449, 1994, wherein an electronic HID ballast used in a low power metal halide (MHL) is described. Secondly, the loading of the push-pull switching power supply is high, but it is seldom to be used as a HID ballast. Thirdly, the power loading of the half bridge/full bridge switching power supply is also high, and it is generally used as a variety of alternating current (AC) electronic ballast. Substantially, an electronic HID ballast may be constructed by a half bridge/full bridge switching power supply using a HID lamp as a load. FIG. 1 is a circuit diagram illustrating a conventional electronic HID ballast using a half bridge/full bridge switching power supply.
In general, an electronic HID ballast can drive the HID lamp in a high frequency range. Before the lamp is turned on, a voltage less than a maximum permissible voltage is provided to the lamp, wherein a pulse voltage is added to the voltage. In general, an open-circuit voltage of the electronic HID ballast has a quasi-square waveform with an over-pulse peak, and the voltage is used as a trigger pulse. FIG. 2 is a waveform diagram illustrating a quasi-square wave having an over-pulse peak as a trigger pulse of the HID lamp. It is noted that different lamp with different type and power has different quasi-square wave with different amplitude and width.
Moreover, the limitation of the HID lamp is that a higher cold-start voltage is required but after lamp is turned on, the hot lamp can not be started again in order to protect the lamp. Therefore, in order to make sure that the lamp can be turned on, a higher amplitude and a wider width of the pulse voltage is preferable. But in order to prolong the operating life of the lamp, the amplitude of the voltage is preferably as low as possible.
In conventional, there are two types of circuit design to provide the pulse voltage. One method is to obtain the pulse voltage from the resonance of the circuit. Another method is to obtain the pulse voltage from a special starting trigger device. For the method using the resonance of the circuit, referring to FIG. 1, a half bridge converter circuit is used an electronic HID ballast, wherein a high frequency square voltage output is obtained in both ends of the HID lamp. In general, the voltage U can be expanded by a Fourier series as illustrated in the following equation (1):
                    U        =                                            ∑              n                        ∞                    ⁢                                    u              n                        ⁢            sin            ⁢                                                  ⁢                          ω              n                        ⁢            t                                              (        1        )            
At the instance of the startup of a cold lamp, the circuit shown in FIG. 11 may be regarded as an open circuit. Moreover, since C>>Cj, the circuit can be simulated by a serial connected circuit RLC as shown in FIG. 3 (wherein R is calculated from the power loss of the circuit). The circuit can be solved by the following equation (2):
                                                        L              ⁢                                                ⅆ                  I                                                  ⅆ                  t                                                      +                                          1                Cj                            ⁢                              ∫                                  I                  ⁢                                      ⅆ                    t                                                                        +            IR                    =                                                    ∑                n                            ∞                        ⁢                                          u                n                            ⁢              sin              ⁢                                                          ⁢                              ω                n                            ⁢              t                                      ⁢                                  ⁢        or                            (        2        )                                                                                    ⅆ                2                            ⁢              I                                      ⅆ                              t                2                                              +                                    R              L                        ⁢                                          ⅆ                I                                            ⅆ                t                                              +                      I            LCj                          =                                            ∑              n                        ∞                    ⁢                                                                      u                  n                                ⁢                                  w                  n                                            L                        ⁢            sin            ⁢                                                  ⁢                          ω              n                        ⁢            t                                              (        3        )            
When the damped oscillation is considered only, the current I can be expanded by the following equation (4):
                    I        =                                            ∑              n                        ∞                    ⁢                      (                                                            a                  n                                ⁢                sin                ⁢                                                                  ⁢                                  ω                  n                                ⁢                t                            +                                                b                  n                                ⁢                cos                ⁢                                                                  ⁢                                  ω                  n                                ⁢                t                                      )                                              (        4        )            
Therefore, by replacing the current I of equation (4) into equation (3) and using the normalization property of the trigonometric function, the equation (3) can be solved. Therefore, the desired resonance to the frequency ωn of the circuit described by equation (3) is obtained by the following equation (5):
                              ω          n          2                =                  1          LCj                                    (        5        )            
Accordingly, in equation (3), when the resonance current is considered and other order harmonic wave currents are omitted, the resonance current Ir is approximated by the following equation (6):
                              Ir          ≈                      I            n                          =                                            u              n                        R                    ⁢          sin          ⁢                                          ⁢                      ω            n                    ⁢          t                                    (        6        )            
The voltage V between both ends of the lamp is then equal to:
                    V        =                              u            Cj                    =                                    -                              u                L                                      =                                                            -                  L                                ⁢                                                      ⅆ                    I                                                        ⅆ                    t                                                              =                                                                                          Lu                      n                                        ⁢                                          ω                      n                                                        R                                ⁢                cos                ⁢                                                                  ⁢                                  ω                  n                                ⁢                t                                                                        (        7        )            
In a conventional fluorescent electronic ballast, the peak value Vmax of the voltage V between both ends of the lamp is obtained by using the resonance between the circuit loop and the basic resonance voltage (wherein n=1). Therefore, the peak value Vmax of the voltage is equal to:
                    Vmax        =                                            Lu              1                        ⁢            ω                    R                                    (        8        )            
Accordingly, if the power loss R of the circuit is small enough, the starting voltage between both ends of the lamp is high enough. In fact, R is very small, thus the Vmax is much larger than the required voltage. Therefore, the loop of the circuit is designed a little far away from the resonance point of the circuit in order to prevent the Vmax from damaging the circuit by adjusting the capacitance Cj.
Conventionally, the above circuit as shown in FIG. 1 is used as an electronic HID ballast. However, the lamp may be damaged suddenly due to the over-pulse peak of the voltage. The reason why the voltage may damage the lamp may be found in the equations described above. According to equations (6) and (8), at the resonance point, the ratio of the maximum current Imax to the voltage Vmax is equal to:
                              Imax          Vmax                =                  1                      n            ⁢                                                  ⁢                          ω              n                        ⁢            L                                              (        9        )            
When the power of the lamp is high, the voltage Vmax is high enough. When n=1, the ratio of equation (9) is equal to:
                              Imax          Vmax                =                  1                      ω            ⁢                                                  ⁢            L                                              (        10        )            
In general, the voltage Vmax is about 1.2 kV. For a 150 W sodium lamp, Imax is about 15 A if ωL is about 80. However, for a 250 W sodium lamp, Imax is larger than about 20 A. Therefore, at the instance of the starting of the lamp, the high current may damage the lamp. If the starting fails, the over-pulse peak voltage will destroy the loop of the circuit or the lamp. Therefore, to provide the circuit shown in FIG. 1 as an electronic ballast of the HID lamp is not practical. Moreover, if the provided circuit is not in the basic resonance, for example, the resonance is in 3rd order or 5th order, the resonance current can be reduced. But the parameters of the circuit need to be adjusted to match the 3rd or the 5th order resonance condition. The adjustments of the parameters are dependent on the whole circuits and need to be optimized by try and error method. Moreover, the power consumption of the circuit will increased since it is proportional to the order of the resonance. Accordingly, the peak value of the voltage will reduce, and thus the opportunity of failure of the startup will increase.
II. The Conventional Startup Circuit for Triggering the HID Lamp
Hereinafter, a variety of conventional startup circuits for triggering the HID lamp will be described. FIG. 4 is a circuit diagram illustrating a conventional startup circuits for triggering the HID lamp of 4 times of voltage type. In early days, the circuit of FIG. 4 is provided as a iron core ballast or a so-called hot starter. FIG. 5 is a circuit diagram illustrating another conventional startup circuits for trigger the HID lamp. Referring to FIG. 5, however, the stability of the circuit and the operating life of the components of the circuit are also the issues of the circuit.
Moreover, the voltage curve from the startup to the stable working period of a conventional HID lamp is shown in FIG. 6. After the voltage of the lamp reach the working voltage, the current of the lamp must be stabilized at the working current, and the current in each period must be continuously. Moreover, in a half period of the voltage curve, two sub-zero voltages can not be occurred to prevent turning off of the arc of the lamp. Finally, no matter what the waveform of the voltage of the lamp is a sine wave, quasi-sine wave, square wave, quasi-square wave, and no matter what the waveform of the current of the lamp is a sine wave, square wave, saw wave or even a sharp peak wave, the wave peak coefficient should less than 1.8 times of that of the sharp peak wave.
Hereinafter, the generation and the influence of the sound resonance of the HID lamp will be described. The sound resonance issue is a special characteristic of the HID lamp. The sound resonance is caused by the standing wave formed by the superposition of the pressure waves reflected from the lamp tube, wherein the pressure wave is caused by the transmission of the high frequency electrical power in the ion plasma of discharged by the arc of the lamp. Therefore, the arc of the lamp will be influenced by the pressure wave of the sound resonance, and thus the arc and the voltage of the lamp will be unstable. Therefore, the light emitted from the arc of the lamp will flicker, more particularly, the arc of the lamp will be distorted or destroyed, and thus the lamp will be burned out or even the lamp tube will be blown up. Especially, when the discharge lamp tube is spherical, the shape of the lamp is symmetrical and the sound resonance is much easily to take place. In general, when the frequency of the current is in a range of about 10 KHz to about 300 KHz, the sound resonance may occur. As a research result, it is found that in a frequency less than 300 KHz, there is almost no frequency that can provide a stable working condition. Accordingly, in order to prevent the sound resonance issue described above, the following methods are provided. The first method is to operate the lamp in a frequency range far away from the sound resonance frequency. The second method is to add one or more proper low frequency component before the sound resonance occurs to prevent from generating of the sound resonance. The third method is to eliminate the band of frequency of the current of the lamp near the band of frequency of the sound resonance. Accordingly, a preferable method is to add one or more low frequency component to the high frequency current, or to turn on the lamp by using a DC current. Moreover, an external startup circuit and a corresponding protection circuit are also required in the methods described above.
III. Calibration of the Frequency Band of the Power
In general, the HID lamp with different type and different working power has their own stable working frequency band. If the frequency band is not suitable for the lamp, the sound resonance may occur, the arc may flicker, and in some specific frequency bands, the tube may burn out. Therefore, in order to operate the HID lamp in a stable high frequency working condition, the frequency band must be selected according to the type, the working power, the shape, and the gas pressure of the lamp. Moreover, the HID tube pressure has a specific dispersion characteristic, and the working power and the color temperature of the lamp will be effected by the dispersion characteristic. Therefore, the working power must be effectively controlled by the circuit of the ballast. Moreover, the harmonics of the input current of the HID electronic ballast will damage the HID lamp. Since the HID lamp is a non-linear electronic component, the power inverter is operated in a high frequency switching condition, thus the waveform of the input current will be distorted. The higher order harmonic wave can not be eliminated by one or more capacitors connected in parallel. The eliminating of the harmonic wave is generally corresponding to the increasing of the power factor. A power factor correction (PFC) IC may be provided as a PWM power control method, thus the power stability of the ballast and the lamp under different working voltage may be solved. However, for all the conventional PFC IC, the working voltage, the dynamic range and the loading power are limited. For example, FIG. 7 is the block diagram illustrating the circuit of MC34262 of Motorola, FIG. 8 is the block diagram illustrating the circuit of L4981A of ST, and FIG. 9 is the block diagram illustrating the UCC3817 of Texas Instrument (TI). Referring to FIG. 7, the MC34262 is worked in a range of 185V to 265V/250 W, but it is difficult to work in a range of 85V to 265 V/250 W. Therefore, ST developed the L4981A (shown in FIG. 8), wherein the IC can work in a range of 88V to 264V/200 W. In addition, TI developed the UCC3817 (shown in FIG. 9), wherein the dynamic range of the IC is in a range of about 85V to about 270V.
IV. Enhancement of the Driving Power
For all the conventional secondary half bridge power driver IC, the loading power are also limited. For example, FIG. 10 is the block diagram illustrating the circuit of L2155, L2156 or L2159 of IR, FIG. 11 is the block diagram illustrating the circuit of UCC3580 or 3895 of TI. These ICs can only be provided in a limited range of loading power. If the circuit peripheral to these ICs is modified to fit the HID lamp, the cost and the complexity of the circuits will increase. If other simple IC circuit, for example, the one shown in FIG. 12, is used to drive a lamp with power loading of 250W to 400W or even 1000W, the lamp and the IC will often be blown up on starting the lamp.
Alternatively, the circuit using the self-excited oscillation method can also be employed for enhancement of the driving power instead of using the ICs. However, the loading power of the circuit is also limited in a range, for example, below 150W. Although the loading power can be enhanced by using the method of increasing the area of the magnetic core, the stabilization of the power is still an issue. For example, if the power loading of the circuit is 150W, in general, the power will drop during the operation and finally maintain at about 120W. FIG. 13 is a block diagram illustrating another circuit of a conventional power drive circuit. It is noted that more than one inductor is provided in the circuit of FIG. 13. Accordingly, except for the stabilization of the power, the complexity and cost of the circuit are also another issue of the circuit.
Another method to enhance the driving power is to replace the power amplifier from the half bridge inverter to a full bridge inverter, and hence, the primary circuit needs to be changed according to the secondary circuit. Alternatively, a method of using capacitor as the voltage divider to enhance the power can also be provided instead of changing the half-bridge inverter, or even changing the electronic component of the current limit inductor.
V. Protection of the Circuit
In general, during the operation of the HID lamp and the ballast, some of the following problem may occur, for example,                (1) The HID lamp is not electrically connected with the base of the lamp, thus the circuit between the lamp and the ballast is an open circuit.        (2) The lamp is deteriorated and can not be turned on, however, the ballast still applies high voltage pulses to the lamp in an effort to turn it on.        (3) The HID lamp is broken, or short.        (4) The power supply is broken suddenly and then is back on; even though the lamp is still hot and can not be turned on, the ballast continues to apply high voltage pulses to the lamp trying to start it.        
Accordingly, if any one of the problems occurs, the lamp should not be started. However, if the ballast and the lamp are not protected by a protection circuit, the ballast will apply high voltage pulses to try to start the lamp when any one of the problem occurs. Thus, the ballast and the lamp will be damaged, the power consumption will increase, and the electromagnetic interference will also increased. Therefore, a protection circuit for the ballast and the lamp is required.
The protection circuit must be incorporated with the ballast and the lamp and provides the function such as to start the lamp after about 3 to 5 minutes. Therefore, the protection circuit must be controlled by a timing circuit. FIG. 14 is a block diagram illustrating a conventional protection circuit. Referring to FIG. 14, a voltage signal proportional to the resonance voltage is provided through the assistant coil of the current limit inductor. The voltage signal is filtered and rectified to be a DC sampling voltage, the DC sampling voltage is then divided and applied to the base-emitter of the protection transistor 1402. The collector of the transistor 1402 is connected to the triggering pole (leg 2) of the timing IC NE555. The IC NE555 is connected in a single stable state, the leg 6 and leg 7 of NE555 is connected to the timer RC, the output pole (leg 3) is connected to the enabling pole (leg 5) of power driving IC (CD4046). When a low voltage level is applied to the leg 5 of CD4046, CD4046 is normally operated, but when a high voltage level is applied to the leg 5 of CD4046, CD4046 is off. Accordingly, when a low level DC sampling voltage is applied to the transistor 1402, the transistor 1402 is off and NE555 is not triggered, a low voltage level is applied to the leg 3 of CD4046, thus CD4046 is normally operated. When some abnormal situation happens, the DC sampling voltage is increased drastically, the transistor is turned on and NE555 is triggered, a high voltage level is applied to the leg 3 of CD4046, thus CD4046 is off. Therefore, the circuit is protected. The protection time of the circuit is determined by the timer RC, in general, the protection time is set about 5 minutes in order to restart the lamp. Alternatively, the IC 2159 or IC 2156 of IR company can also be provided as the protection circuit. However, it is noted that a variety of the conventional protection circuit can not be directly provided as a protection circuit, but must be modified and incorporated with another external circuit to fit the protection requirement. Therefore, the cost and the complexity of the protection circuit is increased.
Accordingly, an electronic ballast for HID lamp at least including the power amplification, light adjusting, restarting, timing and protection circuits is required.